I think not only size plays a factor, but position.
Here's what I've noticed. The position and the size are both factors, just as the ladies have mentioned, but a large cock can force the air back out of the vagina, while a shorter one will leave space in the vagina for the air to occupy.
Here is are some scenarios. A woman is on all fours and a male is entering her vagina from behind. The male pulls all the way out on some strokes, then enters her again. Regardless of the, except for extremely thin penises, the vagina will gape open for just a second, and air will usually enter.
Here's where size makes a difference. The longer and thicker cock can fill the vagina in length and width, leaving less space for the air to stay in, so it gets forced back out if the large cock goes in all the way. Yet, a smaller cock will not take up as much space, so the air can stay inside. So, the position and technique will affect if the air enters, but a large penis will push the air back out sooner. In the case of the small penis, the air stays in till too much air has entered, the woman changes positions, or the woman tightens up her stomach muscles, laughs, sneezes, etc.... So with a small penis, the air comes back out because of the woman's body, not the because of the small penis.